The present invention relates to an apparatus for replicating a pattern of a mask onto a photoresist film formed on a wafer, in the production of integrated circuit. More particularly, the invention is concerned with an X-ray exposure apparatus adapted to replicate the mask pattern by means of X-ray.
The current tendency of large-scale integration of integrated circuit gives a rise to the demand for further refining of the pattern to be replicated on the wafer. To cope with this demand, a method called reduction projection exposure method, making use of ultraviolet rays or deep ultraviolet ray has been put into practical use. This method permits the refining of width and pitch of the pattern to an order of 2 to 3 .mu.m.
For attaining a larger scale of integration, however, it is necessary to provide the line width of the pattern smaller than 1 .mu.m. With the conventional exposure apparatus using light, however, it is very difficult to achieve the replication of minute pattern on the order of sub micron, because of various problems such as diffraction of light, multiple reflection in the optical system, interference of light and so forth.
Hitherto, an X-ray exposure apparatus has been proposed as an apparatus capable of replicating pattern on the order of sub micron using a soft X-ray of a wavelength ranging between 1 and 50 A.
It is well known that, with this X-ray exposure apparatus, it is possible to effect a replication of a pattern at a high precision, because the diffraction and scattering due to contamination are avoided thanks to the use of the X-ray which has a wavelength much smaller than those of the conventionally used ultraviolet rays and super ultraviolet rays. Although some experimental X-ray exposure apparatus have been proposed up to now, none of them are still far from practical use.
One of these proposed X-ray exposure apparatus incorporates an X-ray generating means whole or a part thereof having an X-ray window is movable vertically, a mask having a predetermined pattern and disposed under the X-ray generating means, a table disposed under the mask and movable in three directions X, Y and Z, as well as in the rotational direction .theta., and a horizontally movable optical system disposed laterally of the X-ray generating means and adapted for obtaining an alignment between the mask and the wafer.
In this type of X-ray exposure apparatus, it is necessary that the X-ray window has a strength sufficiently to withstand a large force produced by the pressure differential between the atmospheric pressure and the high vacuum on the order of 10.sup.-6 Torr generated in the X-ray generating means. It is also necessary that the X-ray window has a size substantially equal to the area of the replication. Therefore, for effecting a replication in one time on a wafer of 3 inches dia., it is necessary that the X-ray window has a diameter of 3 inches. In this case, the beryllium constituting the X-ray window has a thickness of 50 .mu.m at the smallest. The increased thickness of the beryllium in turn increases the rate of attenuation of the X-ray transmitted through the X-ray window. For instance, in the case of the X-ray of AlK.alpha. (wavelength 8A), the X-ray is attenuated by 75% as it passes through the beryllium window of 50 .mu.m thickness. A further increase of the replication area to be processed in one time requires an increase of the thickness of the X-ray window, which in turn increases the rate of attenuation of the X-ray undesirably.
Due to a mechanical restriction, a gap of several mm at the smallest is necessary between the X-ray window and the mask in the state of replication. This gap is filled with atmosphere (air) which also causes an attenuation of the X-ray. More specifically, a 20% attenuation of the X-ray is caused when the gap between the X-ray window and the mask is 2 mm.
Since the X-ray is attenuated largely by the gap between the X-ray window and the mask, only 20% or so of the X-ray generated by the X-ray generator is utilized on the upper surface of the mask. In consequence, the time required for the replication of the pattern is prolonged, resulting in a reduction of through put (number of replication cycle per unit time). Thus, this type of apparatus has only a small productivity and cannot be used as a practical equipment for the mass-production of the product.